Varying frontal thrust spacing in mono-vergent wedges: An insight from analogue models     

PUSPENDU SAHA  SANTANU BOSE  NIBIR MANDAL 《Journal of Earth System Science》2013,122(3):699-714

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Garnetiferous Metabasites from the Sausar Mobile Belt: Petrology, P-T Path and Implications for the Tectonothermal Evolution of the Central Indian Tectonic Zone   总被引：1，自引：0，他引：1  

BHOWMIK  SANTANU KUMAR; ROY  ABHINABA 《Journal of Petrology》2003,44(3):387-420

A suite of garnetiferous amphibolites and mafic granulites occuras small boudins within layered felsic migmatite gneiss in thenorthern part of the Sausar Mobile Belt (SMB), the latter constitutingthe southern component of the Proterozoic Central Indian TectonicZone (CITZ). Although the two types of metabasites are in variousstages of retrogression, textural, compositional and phase equilibriastudies attest to four distinct metamorphic episodes. The earlyprograde stage (M_o) is represented by an inclusion assemblageof hornblende₁ + ilmenite₁ + plagioclase₁ ± quartz andgrowth zoning preserved in garnet. The peak assemblage (M₁)consists of porphyroblastic garnet + clinopyroxene ±quartz ± rutile ± hornblende in mafic granulitesand garnet + quartz + hornblende in amphibolites and stabilizedat pressure–temperature conditions of 9–10 kbarand 750–800°C and 8 kbar and 675°C, respectively.This was followed by near-isothermal decompression (M₂), andpost-decompression cooling (M₃) events. In mafic granulites,the former resulted in the development of early clinopyroxene_2A–hornblende_2A–plagioclase_2Asymplectites at 8 kbar and 775°C (M_2A stage), synchronouswith D₂ and later anhydrous clinopyroxene_2B–plagioclase_2B–ilmenite_2Bsymplectites and coronal assemblages at 7 kbar, 750°C (M_2Bstage) and post-dating D₂. In amphibolites, ilmenite + plagioclase+ quartz ± hornblende symplectites appeared during M₂at 6·4 kbar and 700°C. During M₃, coronal garnet+ clinopyroxene + quartz ± hornblende-bearing symplectitesin metabasic dykes and hornblende₃–plagioclase₃ symplectitesembaying garnet in mafic granulites were formed. P–T estimatesshow near-isobaric cooling from 7 kbar and 750°C to 6 kbarand 650°C during M₃. It is argued that the decompressionin the mafic granulites is not continuous, being punctuatedby a distinct heating (prograde?) event. The latter is alsocoincident with a period of extension, marked by mafic dykeemplacement. The combined P–T path of evolution has aclockwise sense and provides evidence for a major phase of earlycontinental subduction in parts of the CITZ. This was followedby a later continent–continent collision event duringwhich granulites of the first phase became tectonically interleavedwith younger lithological units. This tectonothermal event,of possibly Grenvillian age, marks the final amalgamation ofthe North and the South Indian Blocks along the CITZ to producethe Indian subcontinent. KEY WORDS: Central Indian Tectonic Zone; clockwise P–T path; continental collision; metabasite  相似文献   

Mineral chemistry of tourmaline from Mashak Pahar,South Purulia Shear Zone (SPSZ), eastern Indian Shield     

SANTANU ACHARJEE  JYOTISANKAR RAY  PAYEL DEY  DEBAPRIYA BHATTACHARYYA  MOUSUMI BANERJEE  BASAB CHATTOPADHYAY  SHYAMAL SENGUPTA  A K BHATT  D CHOWDHURY  A K DWIVEDI  SANJOY MAHATO  ARKA RANJAN JANA  P B MAITHANI  P V RAMESH BABU 《Journal of Earth System Science》2016,125(8):1681-1696

The area of investigation at and around Mashak Pahar, Bankura district, West Bengal, India comprises a number of rock types namely: granite gneiss, migmatized quartz tourmaline gneiss, quartz pebble conglomerate, ferruginous quartzite, quartz tourmaline veins (as veins) and graphite schists. Interestingly, the study area lies in the region extending South Purulia Shear Zone (～Tamar–Porapahar Shear Zone) which marks the boundary between two contrasting tectonic blocks of eastern India, namely, the Chhotanagpur Gneissic Terrane (CGC) to the north and Singhbhum Group of rocks to the south. The rocks of the study area are poly-phasedly deformed by three phases of folding, namely, F₁, F₂ and F₃. All the tourmalines are classified to be of ‘Alkali Group’. Chemistry of tourmalines from migmatized quartz tourmaline gneiss and those from quartz tourmaline veins are in conformity with their relation to (earthquake induced) shear system evolution in this terrain. In general, the compositional evolution of tourmaline during prograde metamorphism (～400°–730°C) has been supported by both petrographic and chemical evidences. Assessment of mineral–chemical data of constituent tourmaline grains clearly suggests compositional variations across zonal boundaries within tourmaline that was controlled by changing metamorphic milieu in this terrane. Field and petrographic evidences clearly indicate activation of earlier and later shears in this region accompanied by infiltration of boron and formation of zoned tourmaline crystals.  相似文献   

Petro-tectonic Imprints in the Sapphirine Granulites from Anantagiri, Eastern Ghats Mobile Belt, India   总被引：5，自引：0，他引：5  

SENGUPTA  PULAK; DASGUPTA  SOMNATH; BHATTACHARYA  P. K.; FUKUOKA  M.; CHAKRABORTI  SUBRATA; BHOWMICK  SANTANU 《Journal of Petrology》1990,31(5):971-996

Sapphirine granulite occurring as lenses in charnockite at Anantagiri,Eastern Ghat, India, displays an array of minerals which developedunder different P-T-X conditions. Reaction textures in conjunctionwith mineral chemical data attest to several Fe-Mg continuousreactions, such as

1. spinel+rutile+quartz+MgFe_–1=sapphirine+ilmenite
2. cordierite=sapphirine+quartz+MgFe_–1
3. sapphirine+quartz=orthopyroxene+sillimanite+MgFe_–1
4. orthopyroxene+sapphirine+quartz=garnet+MgFe_–1
5. orthopyroxene+sillimanite=garnet+quartz+MgFe_–1
6. orthopyroxene+sillimanite+quartz+MgFe_–1=cordierite.

Calculated positions of the reaction curves in P-T space, togetherwith discrete P-T points obtained through geothermobarometryin sapphirine granulite and the closely associated charnockiteand mafic granulite, define an anticlockwise P-T trajectory.This comprises a high-T/P prograde metamorphic path which culminatedin a pressure regime of 8?3 kb above 950?C, a nearly isobariccooling (IBC) path (from 950?C, 8?3 kb, to 675?C, 7?5kb) anda terminal decompressive path (from 7?5 to 4?5 kb). Spinel,quartz, high-Mg cordierite, and sapphirine were stabilized duringthe prograde high-T/P metamorphism, followed by the developmentof orthopyroxene, sillimanite, and garnet during the IBC. Retrogradelow-Mg cordierite appeared as a consequence of decompressionin the sapphirine granulite. Deformational structures, reportedfrom the Eastern Ghat granulites, and the available geochronologicaldata indicate that prograde metamorphism could have occurredat 30001?00 and 2500?100 Ma during a compressive orogeny thatwas associated with high heat influx through mafic magmatism. IBC ensued from P_max and was thus a direct consequence of progrademetamorphism. However, in the absence of sufficient study onthe spatial variation in P-T paths and the strain historiesin relation to time, the linkage between IBC and isothermaldecompression (ITD) has remained obscure. A prolonged IBC followedby ITD could be the consequence of one extensional mechanismwhich had an insufficient acceleration at the early stage, orITD separately could be caused by an unrelated extensional tectonism.The complex cooled nearly isobarically from 2500 Ma. It sufferedrapid decompression accompanied by anorthosite and alkalinemagmatism at 1400–1000 Ma.  相似文献